An optical information recording medium is known as the information recording medium (which may be referred to as a “recording medium” or a “medium” hereinafter) having a large capacity, on and from which information can be recorded and reproduced at a high speed. Recording information on and reproducing information from this optical information recording medium is carried out using the fact that a recording material is changed into a state which is distinguishable optically, by heat caused by irradiating the recording material with laser beam. This recording medium has an advantage that it allows random access thereto if necessary, and is excellent in portability, and therefore is of increasing importance.
The optical information recording media which have been proposed include a rewritable medium on which information can be overwritten many times and a write-once medium on which information is written only once. The write-once medium can be produced with ease and provided at a low cost since the number of layers constituting the medium is generally smaller compared to the rewritable medium. Further, the write-once medium is advantageously used for a medium in which data that a user does not want anyone to destroy or erase is written since the data cannot be overwritten. For these reasons, there is large demand for the write-once medium which has long archival life and high reliability, as a medium for archival applications. The write-once medium is used for, for example, recording and storing personal data and visual information by means of a computer, and is used widely in the medical field and the academic field. Further, the write-once medium is also used as an alternative for home video tapes.
Now the performances of the application and the visual information are higher and the market develops quickly, which requires the enhancement of the capacity of the write-once recording medium and production of the write-once recording medium at a lower cost.
The technique of enhancing the capacity of the optical information recoding medium is divided into two main classes. One is a technique for increasing the surface recording density by shortening the wavelength of a light source, increasing an NA of an objective lens, and by a superresolution recording. The other is a technique for increasing the number of recording layers in a thickness direction of the medium so as to increase the total surface recording area. Now, the capacity of the optical information recording medium is increasing by combining the both techniques.
Further, the cost per optical information recording medium depends on many production parameters such as a production apparatus, time required for producing the medium and a yield. In particular, the cost of the medium is largely affected by the cost of the material. For example, a noble metal such as Pt, Pd and Au and a rare-earth element such as Eu, Tb and Pr are known as expensive materials since prospects of them are small. Therefore, these elements are preferably avoided as the material constituting the medium in view of the cost of the medium.
The demand for the write-once medium is increasing rapidly with explosive sales of DVD recorders. This is because the usage wherein the video recorded on a hard disk is stored in the write-once recording medium becomes common. Of course, it is presumed that the same usage will be employed by end users when using the next-generation DVD recorder. Thus, the low-cost write-once recording medium having a large capacity which is suitable for recording high-definition broadcast data is receiving attention. One of such mediums is a Blu-ray Disc (BD).
Some oxide materials have been proposed as the write-once type recording material. For example, the recording material is known of which main component is TeOx that is a lower oxide of Te. In particular, it is known that a Te—O—Pd recording material of which main component is TeOx with Pd dispersed therein makes it possible to obtain large signal amplitude and to give very high reliability (see Patent Document 1). It is considered that the recording mechanism of these Te—O—Pd-based recording material is as follows: The Te—O—Pd film after the film formation is a composite material wherein Te—Pd, Te or Pd is uniformly dispersed as microparticles in TeO2. When the laser beam is applied for recording, Te, Te—Pd and Pd are melted and deposited as larger crystalline particles. Thereby, the optical state is changed and the difference in optical state between the portion irradiated with the laser beam and the portion not irradiated with the laser beam can be detected as signal.
The Te—O—Pd-based recording material, however, contains Pd which is expensive as described above, as a component. For this reason, it is difficult to realize the optical information recording medium of low cost, when using this material.
The recording material of which main component is SbOx has been proposed as another oxide material (Patent Documents 2-5). It is presumed that the recording mechanism of the SbOx-based recording material is the same as that of the Te—O—Pd-based material. That is, the SbOx recording material after the film formation is a composite material wherein Sb is uniformly dispersed as microparticles in amorphous Sb—O, and Sb is melted by irradiation of laser beam to be deposited as larger crystalline particles. The optical state is thereby changed such that the difference in optical state can be detected as signal.
A problem of the optical information recording medium wherein the SbOx-based material is used is that a contrast ratio is small. In Patent Document 2, addition of a material such as PbO, In2O5, SnO, B2O5, CuO, TeO2, SiO, GeO2 is proposed for improving the contrast ratio. However, it was found that a C/N ratio is reduced and the recording and reproduction characteristics are lowered when information is recorded at a recording density higher than that employed in the experimental conditions described in Patent Document 2, according to the experiment conducted by the inventors.
In Patent Document 3, a Te—Ge—Sb—Au—O material wherein Au that is a noble metal is contained in the SbOx-based material is proposed. However, the material described in this document contains gold (Au) and hence it is not suitable for reducing the cost.
Patent Document 4 reports the recording material wherein Te and Ge are added to the SbOx-based material. However, the addition amounts of Te and Ge are not expressly indicated in this document. This document merely mentions that, when the content of the metal or the semi metal to be combined with SbOx is over 50 mol %, the laser power required for whitening is large and such material is not preferable in the case where the light source with small output power is employed. Further, this recording material is used for producing the rewritable medium according to this document.
In patent Document 5, a technique of adding ZnS—SiO2 to an SbInSn-based material. It is found that this technique improves signal quality. However, it was found that a C/N ratio is reduced and the recording and reproduction characteristics are lowered when information is recorded at a recording density higher than that employed in the experimental conditions described in this document, according to the experiment conducted by the inventors.    [Patent Literature 1] WO1998/009823    [Patent Literature 2] Japanese Unexamined Patent Publication (kokai) S52-70809    [Patent Literature 3] Japanese Unexamined Patent Publication (kokai) S61-002594    [Patent Literature 4] Japanese Unexamined Patent Publication (kokai) S59-185048    [Patent Literature 5] Japanese Unexamined Patent Publication (kokai) 2003-182237